The present invention relates to heat exchanger arrangements and more particularly to heat exchanger arrangements utilised in gas turbine engines for cooling fluid flows such as with respect to ventilation air or oil within the engine or for de-icing.
Operation of gas turbine engines is well known and incorporates significant fluid flows including compressed air provided by the compressor fans of that gas turbine engine. This compressed air flow is bled for a number of functional operations and in particular in order to provide through an appropriate heat exchanger cooling of other fluid flows such as the ventilation air in the cabin of an aircraft associated with a gas turbine engine or potentially with respect to fuel or lubricating oil coolers in the engine. The coolant flow, as indicated, is tapped or bled from the engine flows and returned at an appropriate location within the engine to maintain a pressure drop sufficient to provide the necessary cooling function within the heat exchanger with respect to the ventilation air or other fluid flow through that heat exchanger. The ventilation air itself is generally taken from hotter core compressor stages of the engine and so needs cooling at least during certain engine cycles.
Prior Art FIGS. 1 and 2 respectively illustrate a schematic side view of a prior heat exchanger arrangement (FIG. 1) and a plan view (FIG. 2) in the direction of A of the heat exchanger arrangement depicted in Prior Art FIG. 1. Thus, the arrangement 1 includes a fluid flow 2 taken from compressor stage 3 air flow generally after a guide vane 4. The bled fluid flow 2 is regulated by a fan air valve 5 such that the fluid flow passes as a coolant through a heat exchanger 6 which exchanges heat with typically another fluid delivered through ducting 7 (shown in broken line). This other fluid is generally a cooled air flow which may be used as the ventilation air for the cabin of an aircraft associated with a gas turbine engine. The fluid flow 2 having been regulated by the valve 5 and passing through the heat exchanger 6 is exhausted as an exhaust flow 8 out of the heat exchanger 6. The heat exchanger 6 and valve 5 are located within a wall 15 of a housing 14, usually known as a splitter fairing, which is generally part of the core nacelle fixing structure of an engine. The exhausted flow 8 mixes with a ventilation flow 13 in a zone 11 that is located radially inwardly of a core cowling 9 and surrounding the engine. In such circumstances the prior heat exchanger arrangement depicted in Prior Art FIGS. 1 and 2 has a number of disadvantages particularly in relation to increasing the temperature in the zone 11 between the housing incorporated in the heat exchanger 6 and surrounding parts of the engine as well as a necessary large vent exit area 12 to generate the desired pressure drop across the heat exchanger resulting in drag to a main propulsive flow 10 when flow through the heat exchanger 6 is low.
In the above circumstances although dumping of the exhaust flow 8 appears to be a relatively simple procedure, there are a number of problems. It will be understood that the exit area 12 has to be sized to cope with the combined ventilation flow 13 and the highest heat exchanger exhaust flow 8 which means that, typically at cruise, when the heat exchanger is operating at low or zero levels it is not possible to recover thrust from this part of the engine as the vent 12 area is effectively oversized. This over sizing also creates a drag penalty as the vent area 12 acts as an aero dynamic step or discontinuity when it is not passing full flow. It will also be understood that extra heat input into the zone 11 requires considerable shielding and heat resistance cabling for the core mounted systems. It will also be understood that by provision of the valve 5 and therefore switchable nature with regard to the flow through the heat exchanger 6 it is difficult to tune the flow regimes in the event of a fire to ensure extinguishants achieve the required density in all parts of zone 11.